The removal of sulfur oxides and of nitrogen oxides contained in industrial effluents is of increasing importance, particularly in view of acid rain phenomena which are highly detrimental to the environment in industrialized countries and countries proximate thereto.
The main sources of these polluting emissions are the fumes from thermal power plants, industrial furnaces and heaters as well as the effluents from various chemical and refining plants: units for manufacturing sulfuric acid, for calcination of ores, Claus units, catalytic cracking units . . .
Many processes for purifying these effluents have already been proposed and some of them are industrially used but they all have some disadvantages.
Thus the processes of the wet type, using aqueous solutions or suspensions of carbonates, hydroxides or sulfites of alkali or alkaline-earth metals or of ammonium, have the disadvantage of requiring the cooling of the fumes and their subsequent re-heating before discharge to the atmosphere. Moreover, the formed products--sulfites and sulfates--raise problems of waste disposal for processes with external discharge, or of a high cost of treatment for processes with regeneration.
The processes of the dry type, using limestone or dolomite, have also the problem of by-products discharge and generally suffer from insufficient performances.
Some processes of the dry type, using regenerable solid absorbents, have also been proposed. Most of them, as the process with alkalinized alumina (U.S. Pat. No. 2,992,884) or the process with copper oxide (U.S. Pat. No. 3,778,501), require the use of such reducing gases as hydrogen or a mixture of hydrogen with carbon monoxide to regenerate the absorbent. These techniques have the disadvantage of consuming expensive reducing gases, which strongly affects their economy.
The use of regenerable desulfurizing masses, consisting essentially of alumina and oxides of alkaline-earth metals, mainly magnesium oxide, is also disclosed in a series of patents concerning the reduction of sulfur oxides emissions from catalytic cracking units. These solid absorbents, admixed to or deposited on the catalytic cracking catalyst, retain as sulfates the sulfur oxides formed during the combustion of the coke deposited on the catalyst and are regenerated in the cracking step by reaction with the present hydrocarbons, with formation of hydrogen sulfide.
U.S. Pat. No. 3,855,031 may be considered as the basic patent in this field. Many patents of improvement, as FR Pat. No. 2,512,050 and U.S. Pat. Nos. 4,240,899 and 4,300,997, claim an improvement in collecting sulfur oxides by these absorbents consisting of incorporating therewith small amounts of compounds of group VIII noble metals or rare earths, acting in particular as promoters of the oxidation of sulfur dioxide to sulfur trioxide. The mere principle of this technique limits its application to the reduction of sulfur oxides emissions from catalytic cracking units.
Another patent family discloses masses absorbing sulfur oxides, containing mainly metal oxides regenerable by reaction with hydrogen sulfide. Thus, U.S. Pat. No. 3,755,535 claims the use of absorbents having aluminum oxide and/or magnesium oxide as main constituents for removing only sulfur trioxide. U.S. Pat. No. 4,426,365 discloses the use of alumina containing alkali or alkaline-earth metal oxides for removing nitrogen oxides and optionally sulfur oxides at a temperature of 85.degree.-200.degree. C. However, in these conditions, the efficiency of the sulfur dioxide removal is very limited, particularly with so-called regenerated absorbents and no performance rate is given for magnesium oxide U.S. Pat. No. 4,283,380 states the difficulty of regenerating, by reduction with hydrogen sulfide, absorbents mainly consisting of alkalized aluminas, by specifically claiming aluminas free of alkaline substances. Unhappily the reaction velocity of sulfur oxides, particularly sulfur dioxide, with such absorbents is low.